1. Field of the Invention
This invention relates to a recording sheet and, more particularly, to a recording sheet capable of forming color when in contact with an almost colorless compound (hereinafter referred to as a color former).
2. Description of the Prior Art
Recording materials utilizing a coloration reaction between a color former and an adsorptive or reactive compound (hereinafter, referred to as a color developer) such as clay (e.g., acid clay, activated clay, attapulgite, zeolite, bentonite, kaolin, etc.), an organic acid (e.g., salicyclic acid, tannic acid, gallic acid, a phenolic compound, etc.), metal salt thereof, or an acidic polymer such as a phenol-formaldehyde resin are well known.
The color former is a material capable of providing an electron or accepting a proton from an acid to form a color, and the color developer is a material capable of accepting an electron or providing a proton.
As specific recording materials utilizing this phenomenon, there are pressure-sensitive copying papers (such as disclosed in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250, Japanese Patent Applicaton (OPI) Nos. 28,411/74, (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and 44,009/75), heat-sensitive recording papers (such as disclosed in Japanese Patent Publication No. 4,160/68, U.S. Pat. No. 2,939,009, etc.), electro-recording papers, etc. Further, there is also known a printing method of supplying a color former-containing ink to a color developer-coated sheet through a medium such as a stencil to obtain colored images (German OLS No. 1,939,624). In the case of utilizing the above-described phenomenon, a means is required to bring the color former and the color developer into contact with each other, such as application of pressure through a pencil or a typewriter key, heat, or the like.
As the most typical embodiment of recording materials, there can be illustrated pressure-sensitive copying paper. The color former layer of pressure-sensitive copying paper can be obtained by dissolving a color former in a solvent, dispersing the resulting solution in a binder or encapsulating it, and coating the composition on a support such as paper, a plastic film, resin-coated paper, etc.
On the other hand, a color developer sheet is obtained by dissolving or dispersing a color developer in a medium like water together with a binder, and coating on a support. In general, the color former and the color developer are coated on the same or opposite sides of a support, or on facing surfaces of different supports. Upon crushing the microcapsules by handwriting or typewriting the color former in the microcapsules is released and comes into contact with the color developer to form color. Thus, a record can be obtained according to the applied pressure.
As has been described above, many materials have been used as the color developer. However, clays, i.e., activated clay, acid clay, etc. are at present often employed as color developers and have the defect that they provide a color with poor light stability or water resistance. On the other hand, a phenol monomer or a phenol-formaldehyde resin has the defect as a color developer that the coated surface is liable to yellow.
It has been discovered that polyvalent metal salts of aromatic carboxylic acids are effective as a color developer without such defects (U.S. Pat. No. 3,934,070). That is, when the polyvalent metal salts of aromatic carboxylic acids are used as a color developer, the light resistance of the resulting color is excellent, the formed color will not disappear when in contact with water, and the color does not readily yellow even when exposed to light. However, there still remain several defects and improvements are needed.
For example, from the standpoint of performance, the coated surface is difficult to smooth in the case of coating a coating solution containing a comparatively low concentration of solids using air-knife coating and, color stain due to friction is liable to occur. In addition, an unsmooth surface produces unevenly colored images with poor resolving power upon handwriting or typewriting. Also, in production, there is the defect that solid ingredients must be incorporated in high concentrations so as to minimize, upon coating, the amount of water removed.